U.S. patent number 8,445,980 [Application Number 13/216,453] was granted by the patent office on 2013-05-21 for memory element and memory device.
This patent grant is currently assigned to Sony Corporation. The grantee listed for this patent is Kazuhiro Bessho, Yutaka Higo, Masanori Hosomi, Hiroyuki Ohmori, Hiroyuki Uchida, Kazutaka Yamane. Invention is credited to Kazuhiro Bessho, Yutaka Higo, Masanori Hosomi, Hiroyuki Ohmori, Hiroyuki Uchida, Kazutaka Yamane.
United States Patent |
8,445,980 |
Higo , et al. |
May 21, 2013 |
Memory element and memory device
Abstract
There is disclosed a memory element which includes a layered
structure. The layered structure includes a memory layer that has a
magnetization perpendicular to a film face; a magnetization-fixed
layer having magnetization perpendicular to the film face; an
insulating layer provided between the memory layer and the
magnetization-fixed layer; and a cap layer provided at a face side,
which is opposite to the insulating layer-side face, of the memory
layer, in which an electron that is spin-polarized is injected in a
lamination direction of the layered structure, and thereby the
magnetization direction of the memory layer varies and a recording
of information is performed, a magnitude of an effective
diamagnetic field which the memory layer receives is smaller than a
saturated magnetization amount of the memory layer, and at least a
face, which comes into contact with the memory layer, of the cap
layer is formed of a Ta film.
Inventors: |
Higo; Yutaka (Kanagawa,
JP), Hosomi; Masanori (Tokyo, JP), Ohmori;
Hiroyuki (Kanagawa, JP), Bessho; Kazuhiro
(Kanagawa, JP), Yamane; Kazutaka (Kanagawa,
JP), Uchida; Hiroyuki (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Higo; Yutaka
Hosomi; Masanori
Ohmori; Hiroyuki
Bessho; Kazuhiro
Yamane; Kazutaka
Uchida; Hiroyuki |
Kanagawa
Tokyo
Kanagawa
Kanagawa
Kanagawa
Tokyo |
N/A
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
Sony Corporation (Tokyo,
JP)
|
Family
ID: |
45770087 |
Appl.
No.: |
13/216,453 |
Filed: |
August 24, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120056284 A1 |
Mar 8, 2012 |
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Foreign Application Priority Data
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Sep 8, 2010 [JP] |
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2010-200984 |
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Current U.S.
Class: |
257/421;
257/E29.323 |
Current CPC
Class: |
G11C
11/1675 (20130101); G11C 11/16 (20130101); G11C
11/161 (20130101) |
Current International
Class: |
H01L
29/82 (20060101); H01L 43/00 (20060101) |
Field of
Search: |
;257/421,E29.323 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2003-17782 |
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Jan 2003 |
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JP |
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2008-227388 |
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Sep 2008 |
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JP |
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Other References
L Berger, "Emission of spin waves by a magnetic multilayer
traversed by a current", Physical Review B, vol. 54, No. 13, 1996.
cited by applicant .
J.C. Slonczewski, "Current-driven excitation of magnetic
multilayers", Journal of Magnetism and Magnetic Material, 159,
L1-L7, 1996. cited by applicant .
Albert, et al., "Spin-polarized current switching of a Co thin film
nanomagnet", Applied Physics Letters, vol. 77, No. 23, 2000. cited
by applicant.
|
Primary Examiner: Blum; David S
Attorney, Agent or Firm: K&L Gates LLP
Claims
The application is claimed as follows:
1. A memory element, comprising: a layered structure including, a
memory layer that has a magnetization perpendicular to a film face
and a magnetization direction that varies corresponding to
information, a magnetization-fixed layer that has a magnetization
indirection perpendicular to the film face and becomes a reference
for the information stored in the memory layer, an insulating layer
provided between the memory layer and the magnetization-fixed layer
and formed of a non-magnetic material, and a cap layer provided at
a face side opposite to the insulating layer-side face of the
memory layer, wherein: an electron that is spin-polarized is
injected in a lamination direction of the layered structure, and
thereby the magnetization direction of the memory layer varies and
a recording of information is performed with respect to the memory
layer, a magnitude of an effective diamagnetic field which the
memory layer receives is smaller than a saturated magnetization
amount of the memory layer, and at least a face of the cap layer
that comes into contact with the memory layer is formed of a Ta
film.
2. The memory element according to claim 1, wherein the memory
layer includes Co--Fe--B.
3. The memory element according to claim 2, wherein the cap layer
comprises a non-magnetic film laminated on the Ta film that comes
into contact with the memory layer.
4. The memory element according to claim 3, wherein the Ta film
that comes into contact with the memory layer has a film thickness
of 5 nm or less.
5. A memory device, comprising: a memory element that retains
information through a magnetization state of a magnetic material;
and two kinds of interconnects that intersect with each other,
wherein the memory element includes a layered structure including,
a memory layer that has a magnetization perpendicular to a film
face and a magnetization direction that varies corresponding to
information, a magnetization-fixed layer that has a magnetization
direction perpendicular to the film face and becomes a reference
for the information stored in the memory layer, an insulating layer
provided between the memory layer and the magnetization-fixed layer
and formed of a non-magnetic material, and a cap layer provided at
a face side opposite to the insulating layer-side face of the
memory layer, wherein: an electron that is spin-polarized is
injected in a lamination direction of the layered structure, and
thereby the magnetization direction of the memory layer varies and
a recording of information is performed with respect to the memory
layer, a magnitude of an effective diamagnetic field which the
memory layer receives is smaller than a saturated magnetization
amount of the memory layer, at least a face of the cap layer that
comes into contact with the memory layer is formed of a Ta, the
memory element is disposed between the two kinds of interconnects,
and a current flows to the memory element in the lamination
direction through the two kinds of interconnects, and thereby a
spin-polarized electron is injected into the memory element.
6. The memory element according to claim 2, wherein a composition
of Co--Fe--B in the memory layer is
(Co.sub.xFe.sub.y).sub.100-zB.sub.z where 0.ltoreq.x.ltoreq.40,
60.ltoreq.y.ltoreq.100, and 0<z.ltoreq.30.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
The present application claims priority to Japanese Priority Patent
Application JP 2010-200984 filed in the Japan Patent Office on Sep.
8, 2010, the entire contents of which are hereby incorporated by
reference.
BACKGROUND
The present application relates to a memory element that includes a
memory layer that stores the magnetization state of a ferromagnetic
layer as information and a magnetization-fixed layer in which a
magnetization direction is fixed, and that changes the
magnetization direction of the memory layer by flowing a current,
and a memory device having the memory element.
In an information device such as a computer, a highly dense DRAM
that operates at a high speed has been widely used as a random
access memory.
However, the DRAM is a volatile memory in which information is
erased when power is turned off, such that a non-volatile memory in
which the information is not erased is desirable.
In addition, as a candidate for the non-volatile memory, a magnetic
random access memory (MRAM) in which the information is recorded by
magnetization of a magnetic material has attracted attention and
therefore has been developed.
The MRAM makes a current flow to two kinds of address interconnects
(a word line and a bit line) that are substantially perpendicular
to each other, respectively, and inverts the magnetization of a
magnetic layer of a magnetic memory element, which is located at an
intersection of the address interconnects, of the memory device by
using a current magnetic field generated from each of the address
interconnects, and thereby performs the recording of
information.
A schematic diagram (perspective view) of a general MRAM is shown
in FIG. 10.
A drain region 108, a source region 107, and a gate electrode 101,
which make up a selection transistor that selects each memory cell,
are formed at portions separated by an element separation layer 102
of a semiconductor substrate 110 such as a silicon substrate,
respectively.
In addition, a word line 105 extending in the front-back direction
in the drawing are provided at an upper side of the gate electrode
101.
The drain region 108 is formed commonly to left and right selection
transistors in the drawing, and an interconnect 109 is connected to
the drain region 108.
In addition, magnetic memory elements 103, each having a memory
layer whose magnetization direction is inverted, are disposed
between the word line 105 and bit lines 106 that are disposed at an
upper side in relation to the word line 105 and extend in the
left-right direction. These magnetic memory elements 103 are
configured, for example, by a magnetic tunnel junction element (MTJ
element).
In addition, the magnetic memory elements 103 are electrically
connected to the source region 107 through a horizontal bypass line
111 and a vertical contact layer 104.
When a current is made to flow to the word line 105 and the bit
lines 106, a current magnetic field is applied to the magnetic
memory element 103 and thereby the magnetization direction of the
memory layer of the magnetic memory element 103 is inverted, and
therefore it is possible to perform the recording of
information.
In addition, in regard to a magnetic memory such as the MRAM, it is
necessary for the magnetic layer (memory layer) in which the
information is recorded to have a constant coercive force in order
to stably retain the recorded information.
On the other hand, it is necessary to make a certain amount of
current flow to the address interconnect to order to rewrite the
recorded information.
However, along with miniaturization of the element making up the
MRAM, the address interconnect becomes thin, such that it is
difficult to flow a sufficient current.
Therefore, as a configuration capable of realizing the
magnetization inversion with a relatively small current, a memory
having a configuration using a magnetization inversion by spin
injection has attracted attention (for example, refer to Japanese
Unexamined Patent Application Publication Nos. 2003-17782 and
2008-227388, and a specification of U.S. Pat. No. 6,256,223, Phys.
Rev. B, 54.9353 (1996), and J. Magn. Mat., 159, L1 (1996)).
Magnetization inversion by the spin injection means that a spin
polarized electron after passing through a magnetic material is
injected into the other magnetic material, and thereby
magnetization inversion is caused in the other magnetic
material.
For example, when a current is made to flow to a giant
magnetoresistive effect element (GMR element) or a magnetic tunnel
junction element (MTJ element) in a direction perpendicular to a
film face, the magnetization direction of at least a part of the
magnetic layer of this element may be inverted.
In addition, magnetization inversion by spin injection has an
advantage in that even when the element becomes minute, it is
possible realize the magnetization inversion without increasing the
current.
A schematic diagram of the memory device having a configuration
using magnetization inversion by the above-described spin injection
is shown in FIGS. 11 and 12. FIG. 11 shows a perspective view, and
FIG. 12 shows a cross-sectional view.
A drain region 58, a source region 57, and a gate electrode 51 that
make up a selection transistor for the selection of each memory
cell are formed, respectively, in a semiconductor substrate 60 such
as a silicon substrate at portions isolated by an element isolation
layer 52. Among them, the gate electrode 51 also functions as a
word line extending in the front-back direction in FIG. 11.
The drain region 58 is formed commonly to left and right selection
transistors in FIG. 11, and an interconnect 59 is connected to the
drain region 58.
A memory element 53 having a memory layer in which a magnetization
direction is inverted by spin injection is disposed between the
source region 57 and bit lines 56 that are disposed in an upper
side of the source region 57 and extended in the left-right
direction in FIG. 11.
This memory element 53 is configured by, for example, a magnetic
tunnel junction element (MTJ element). The memory element 53 has
two magnetic layers 61 and 62. In the two magnetic layers 61 and
62, one side magnetic layer is set as a magnetization-fixed layer
in which the magnetization direction is fixed, and the other side
magnetic layer is set as a magnetization-free layer in which the
magnetization direction varies, that is, a memory layer.
In addition, the memory element 53 is connected to each bit line 56
and the source region 57 through the upper and lower contact layers
54, respectively. In this manner, when a current is made to flow to
the memory element 53, the magnetization direction of the memory
layer may be inverted by spin injection.
In the case of the memory device having a configuration using
magnetic inversion by this spin injection, it is possible to make
the structure of the device simple compared to the general MRAM
shown in FIG. 10, and therefore it has a characteristic in that
high densification becomes possible.
In addition, when magnetization inversion by the spin injection is
used, there is an advantage in that even as miniaturization of the
element proceeds, the write current is not increased, compared to
the general MRAM performing magnetization inversion by an external
magnetic field.
SUMMARY
However, in the case of the MRAM, a write interconnect (word line
or bit line) is provided separately from the memory element, and
the writing of information (recording) is performed using a current
magnetic field generated by flowing a current to the write
interconnect. Therefore, it is possible to make a sufficient amount
of current necessary for the writing flow to the write
interconnect.
On the other hand, in the memory device having a configuration
using magnetization inversion by spin injection, it is necessary to
invert the magnetization direction of the memory layer by
performing spin injection using a current flowing to the memory
element.
Since the writing (recording) of information is performed by
directly flowing a current to the memory element as described
above, a memory cell is configured by connecting the memory element
to a selection transistor to select a memory cell that performs the
writing. In this case, the current flowing to the memory element is
restricted to a current magnitude capable of flowing to the
selection transistor (a saturation current of the selection
transistor).
Therefore, it is necessary to perform writing with a current equal
to or less than the saturation current of the selection transistor,
and therefore it is necessary to diminish the current flowing to
the memory element by improving spin injection efficiency.
In addition, to increase the read-out signal strength, it is
necessary to secure a large magnetoresistance change ratio, and to
realize this, it is effective to adopt a configuration where an
intermediate layer that comes into contact with both sides of the
memory layer is set as a tunnel insulating layer (tunnel barrier
layer).
In this way, in a case where the tunnel insulating layer is used as
the intermediate layer, the amount of current flowing to the memory
element is restricted to prevent the insulation breakdown of the
tunnel insulating layer. From this viewpoint, it is also necessary
to restrict the current at the time of spin injection.
Since such a current value is proportional to a film thickness of
the memory layer and is proportional to the square of the
saturation magnetization of the memory layer, it may be effective
to adjust these (film thickness and saturated magnetization) to
decrease such a current value (for example, refer to F. J. Albert
et al., Appl. Phy. Lett., 77, 3809 (2000)).
For example, in U.S. Pat. No. 7,242,045, the fact that when the
amount of magnetization (Ms) of the recording material is
decreased, the current value may be diminished is disclosed.
However, on the other hand, if the information written by the
current is not stored, a non-volatile memory is not realized. That
is, it is necessary to secure a stability (thermal stability)
against the thermal fluctuation of the memory layer.
In the case of the memory element using magnetization inversion by
spin injection, since the volume of the memory layer becomes small,
simply considered the thermal stability tends to decrease, compared
to the MRAM in the related art.
When thermal stability of the memory layer is not secured, the
inverted magnetization direction re-inverts by heating, and this
leads to writing error.
In addition, in a case where high capacity of the memory element
using magnetization inversion by the spin injection is advanced,
the volume of the memory element becomes smaller, such that the
securing of the thermal stability becomes an important problem.
Therefore, in regard to the memory element using the magnetization
inversion by spin injection, thermal stability is a very important
characteristic.
Therefore, to realize a memory element having a configuration where
the magnetization direction of the memory layer as a memory is
inverted by spin injection, it is necessary to diminish the current
necessary for the magnetization inversion by spin injection to a
value equal to or less than the saturation current of the
transistor, and thereby securing thermal stability for retaining
written information reliably.
As described above, to diminish the current necessary for the
magnetization inversion by spin injection, diminishing the
saturated magnetization amount Ms of the memory layer, or making
the memory layer thin may be considered. For example, as is the
case with U.S. Pat. No. 7,242,045, it is effective to use a
material having a small saturated magnetization amount Ms as the
material for the memory layer. However, in this way, in a case
where the material having the small saturated magnetization amount
Ms is simply used, it is difficult to secure thermal stability for
reliably retaining information.
Therefore, in this application, it is desirable to provide a memory
element capable of improving thermal stability while diminishing
the write current, and a memory device with the memory element.
According to an embodiment, there is provided a memory element
including a layered structure. The layered structure includes a
memory layer that has a magnetization perpendicular to a film face
and a magnetization direction thereof varies corresponding to
information, a magnetization-fixed layer that has a magnetization
that is perpendicular to the film face and becomes a reference for
the information stored in the memory layer, an insulating layer
that is provided between the memory layer and the
magnetization-fixed layer and is formed of a non-magnetic material,
and a cap layer that is provided at a face side, which is opposite
to the insulating layer-side face, of the memory layer. An electron
that is spin-polarized is injected in a lamination direction of the
layered structure, and thereby the magnetization direction of the
memory layer varies and a recording of information is performed
with respect to the memory layer, a magnitude of an effective
diamagnetic field which the memory layer receives is smaller than a
saturated magnetization amount of the memory layer, and at least a
face, which comes into contact with the memory layer, of the cap
layer is formed of a Ta film.
In addition, the memory layer may include Co--Fe--B.
In addition, the cap layer may be configured in such a manner that
a non-magnetic film is laminated on the Ta film that comes into
contact with the memory layer.
In addition, the Ta film, which comes into contact with the memory
layer, of the cap layer may have a film thickness of 5 nm or
less.
According to another embodiment, there is provided a memory device
including a memory element that retains information through the
magnetization state of a magnetic material, and two kinds of
interconnects that intersect each other, wherein the memory element
has the configuration of the above-described memory element
according to the embodiment, the memory element is disposed between
the two kinds of interconnects, and a current flows to the memory
element in the lamination direction through the two kinds of
interconnects, and thereby a spin-polarized electron is injected
into the memory element.
According to the configuration of the memory element of the
embodiment, a memory layer that retains information through a
magnetization state of a magnetic material is provided, a
magnetization-fixed layer is provided over the memory layer through
an intermediate layer, the intermediate layer is formed of an
insulating material, an electron that is spin-polarized is injected
in a lamination direction and thereby the magnetization direction
of the memory layer is changed and a recording of information is
performed with respect to the memory layer, and therefore it is
possible to perform the recording of the information by flowing a
current in the lamination direction and by injecting a
spin-polarized electron.
In addition, the magnitude of an effective diamagnetic field which
the memory layer receives is smaller than a saturated magnetization
amount of the memory layer, such that the diamagnetic field which
the memory layer receives decreases, and therefore it is possible
to diminish the amount of the write current necessary for inverting
the magnetization direction of the memory layer.
On the other hand, it is possible to diminish the amount of the
write current even when the saturated magnetization amount of the
memory layer is not diminished, such that the saturated
magnetization amount of the memory layer becomes sufficient, and it
is possible to sufficiently secure thermal stability of the memory
layer.
In addition, due to the structure of the cap layer, the Ta film
comes into contact with the memory layer, such that perpendicular
magnetic anisotropy is applied to the memory layer.
In addition, according to the configuration of the memory device of
the embodiment, the memory element is disposed between the two
kinds of interconnects, and a current flows to the memory element
in the lamination direction through the two kinds of interconnects,
and thereby a spin-polarized electron is injected into the memory
element. Therefore, it is possible to perform the recording of
information by a spin injection by flowing a current in the
lamination direction of the memory element through the two kinds of
interconnects.
In addition, even when the saturated magnetization amount is not
diminished, it is possible to diminish the amount of the write
current of the memory element, such that it is possible to stably
retain the information recorded in the memory element and it is
possible to diminish the power consumption of the memory
device.
According to the embodiments, even when the saturated magnetization
amount of the memory layer is not diminished, the amount of the
write current of the memory element may be diminished, such that
the thermal stability representing the information retaining
ability is sufficiently secured, and it is possible to configure a
memory element excellent in a characteristic balance. Therefore, it
is possible to realize a memory device that operates stably with
high reliability.
In addition, at least a face, which comes into contact with the
memory layer, of the cap layer is formed of a Ta film, such that
perpendicular magnetization is applied to the memory layer.
Therefore, it is advantageous in an aspect of realizing a
perpendicular magnetization-type MTJ.
In addition, the write current is diminished, such that it is
possible to diminish power consumption while performing the writing
into the memory element. Therefore, it is possible to diminish the
power consumption of the entirety of the memory device.
Additional features and advantages are described herein, and will
be apparent from the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is an explanatory view illustrating a schematic
configuration of a memory device according to an embodiment;
FIG. 2 is a cross-sectional view illustrating a memory element
according the embodiment;
FIG. 3 is a diagram illustrating a relationship between an amount
of Co of a memory layer of 0.09.times.0.18 .mu.n size and an
inversion current density;
FIG. 4 is a diagram illustrating a relationship between an amount
of Co of a memory layer of 0.09.times.0.18 .mu.m size and an index
of thermal stability;
FIG. 5 is a diagram illustrating a relationship between an amount
of Co of a memory layer of 50 nm .phi. size and an index of thermal
stability;
FIGS. 6A and 6B are explanatory views illustrating a layered
structure and a measurement result of experiment 5 according to the
embodiment;
FIGS. 7A and 7B are explanatory views illustrating a layered
structure and a measurement result of experiment 5 according to the
embodiment;
FIGS. 8A and 8B are explanatory views illustrating a layered
structure and a measurement result of experiment 5 according to the
embodiment;
FIGS. 9A to 9D are explanatory views illustrating a layered
structure and a measurement result of experiment 5 according to the
embodiment;
FIG. 10 is a perspective view schematically illustrating a
configuration of an MRAM in the related art.
FIG. 11 is an explanatory view illustrating a schematic
configuration of a memory device using a magnetic inversion by a
spin injection; and
FIG. 12 is a cross-sectional view of a memory device of FIG.
11.
DETAILED DESCRIPTION
Embodiments of the present application will be described below in
detail with reference to the drawings.
1. Outline of Memory Element of Embodiment
2. Configuration of Embodiment
3. Experiment
1. Outline of Memory Element of Embodiment
First, outline of a memory element of an embodiment according to
the present application will be described.
The embodiment according to the present application performs the
recording of information by inverting a magnetization direction of
a memory layer of a memory element by the above-described spin
injection.
The memory layer is formed of a magnetic material such as
ferromagnetic layer, and retains information through the
magnetization state (magnetization direction) of the magnetic
material.
It will be described later in detail, but the memory element has a
layered structure whose example is shown in FIG. 2, and includes a
memory layer 17 and a magnetization-fixed layer 15 as two magnetic
layers, and an insulating layer 16 (tunnel insulating layer) as an
intermediate layer provided between the two magnetic layers. In
addition, a cap layer 18 is provided at a face side, which is
opposite to the insulating layer 16 side face, of the memory layer
17.
The memory layer 17 has a magnetization perpendicular to a film
face and a magnetization direction varies corresponding to
information.
The magnetization-fixed layer 15 has a magnetization that is a
reference for the information stored in the memory layer 17 and is
perpendicular to the film face.
The insulating layer 16 is formed of a non-magnetic material and is
provided between the memory layer 17 and the magnetization-fixed
layer 15. For example, the insulating layer 16 is formed of an
oxide film such as MgO.
Spin-polarized electrons are injected in the lamination direction
of a laminated structure having the magnetization-fixed layer 15,
the insulating layer 16, the memory layer 17, and the cap layer 18,
and the magnetization direction of the memory layer 17 is changed
and thereby information is recorded in the memory layer 17.
In this embodiment, at least a face, which comes into contact with
the memory layer 17, of the cap layer 18 formed at an upper side of
the memory layer 17 is formed of a Ta film.
A basic operation for inverting the magnetization direction of the
magnetic layer (memory layer 17) by spin injection is to make a
current of a threshold value or greater flow to the memory element
including a giant magnetoresistive effect element (GMR element) or
a tunnel magnetoresistive effect element (MTJ element) in a
direction perpendicular to a film face. At this time, the polarity
(direction) of the current depends on the inverted magnetization
direction.
In a case where a current having an absolute value less than the
threshold value is made to flow, magnetization inversion does not
occur.
A threshold value Ic of a current, which is necessary when the
magnetization direction of the magnetic layer is inverted by spin
injection, is expressed by the following equation (1).
Ic=A.alpha.MsVHd/2.eta. (1)
Here, A: constant, .alpha.: spin braking constant, spin injection
efficiency, Ms: saturated magnetization amount, V: volume of memory
layer, and Hd: effective diamagnetic field.
As expressed by the equation (1), a threshold value of a current
may be set to an arbitrary value by controlling the volume V of the
magnetic layer, the saturated magnetization Ms of the magnetic
layer, and the spin injection efficiency .eta., and the spin
braking constant .alpha.,
In this embodiment, the memory element includes the magnetic layer
(memory layer 17) that is capable of retaining information through
the magnetization state, and the magnetization-fixed layer 15 whose
magnetization direction is fixed.
The memory element has to retain written information so as to
function as a memory. This is determined by a value of an index
.DELTA.(=KV/k.sub.BT) of thermal stability as an index of ability
of retaining information. The above-described .DELTA. is expressed
by the following equation (2).
.DELTA.=KV/k.sub.BT=MsVH.sub.k(1/2k.sub.BT) (2)
Here, H.sub.k: effective anisotropy field, k.sub.B: Boltzmann's
constant, T: temperature, Ms: saturated magnetization amount, and
V: volume of memory layer.
The effective anisotropy field H.sub.k receives an effect by a
shape magnetic anisotropy, an induced magnetic anisotropy, and a
crystal magnetic anisotropy, or the like, and when assuming a
coherent rotation model of a single domain, the effective
anisotropy field becomes the same as the coercive force.
The index .DELTA. of the thermal stability and the threshold value
Ic of the current are often in a trade-off relationship. Therefore,
compatibility of these becomes an issue to retain the memory
characteristic.
In regard to the threshold value of the current that changes the
magnetization state of the memory layer 17, actually, for example,
in a TMR element in which the thickness of the memory layer 17 is 2
nm, and a planar pattern is substantially an elliptical shape of
100 nm.times.150 nm, a threshold value +Ic of a positive side is
+0.5 mA, a threshold value -Ic of a negative side is -0.3 mA, and a
current density at this time is substantially 3.5.times.10.sup.6
A/cm.sup.2. These substantially correspond to the above-described
equation (1).
On the contrary, in a common MRAM that performs a magnetization
inversion using a current magnetic field, a write current of
several mA or more is necessary.
Therefore, in the case of performing magnetization inversion by
spin injection, the threshold value of the above-described write
current becomes sufficiently small, such that this is effective for
diminishing power consumption of an integrated circuit.
In addition, an interconnect (interconnect 105 of FIG. 10) for the
generation of the current magnetic field, which is necessary for a
common MRAM, is not necessary, such that in regard to the degree of
integration, it is advantageous compared to the common MRAM.
In the case of performing the magnetization inversion by spin
injection, since the writing (recording) of information is
performed by directly flowing a current to the memory element, to
select a memory cell that performs the writing, the memory element
is connected to a selection transistor to construct the memory
cell.
In this case, the current flowing to the memory element is
restricted to a current magnitude (saturated current of the
selection transistor) that can be made to flow to the selection
transistor.
To make the threshold value Ic of a current of the magnetization
inversion by the spin injection smaller than the saturated current
of the selection transistor, as can be seen from the equation (1),
it is effective to diminish the saturated magnetization amount Ms
of the memory layer 17.
However, in the case of simply diminishing the saturated
magnetization amount Ms (for example, U.S. Pat. No. 7,242,045), the
thermal stability of the memory layer 17 is significantly
deteriorated, and therefore it is difficult for the memory element
to function as a memory.
To construct the memory, it is necessary that the index .DELTA. of
the thermal stability is equal to or greater than a magnitude of a
certain degree.
The present inventors have made various studies, and as a result
thereof, they have found that when for example, a composition of
Co--Fe--B is selected as the ferromagnetic layer making up the
memory layer 17, the magnitude of the effective diamagnetic field
(Meffective) which the memory layer 17 receives becomes smaller
than the saturated magnetization amount Ms of the memory layer
17.
By using the above-described ferromagnetic material, the magnitude
of the effective diamagnetic field which the memory layer 17
receives becomes smaller than the saturated magnetization amount Ms
of the memory layer 17.
In this manner, it is possible to make the diamagnetic field which
the memory layer 17 receives small, such that it is possible to
obtain an effect of diminishing the threshold value Ic of a current
expressed by the equation (1) without deteriorating the thermal
stability .DELTA. expressed by the equation (2).
In addition, the present inventors have found that Co--Fe--B
magnetizes in a direction perpendicular to a film face within a
restricted composition range of the selected Co--Fe--B composition,
and due to this, it is possible to secure a sufficient thermal
stability even in the case of an extremely minute memory element
capable of realizing Gbit class capacity.
Therefore, in regard to a spin injection-type memory of the Gbit
class, it is possible to make a stable memory in which information
may be written with a low current in a state where the thermal
stability is secured.
In this embodiment, it is configured such that the magnitude of the
effective diamagnetic field which the memory layer 17 receives is
made to be less than the saturated magnetization amount Ms of the
memory layer 17, that is, a ratio of the magnitude of the effective
diamagnetic field with respect to the saturated magnetization
amount Ms of the memory layer 17 becomes less than 1.
In addition, a magnetic tunnel junction (MTJ) element is configured
by using a tunnel insulating layer (insulating layer 16) formed of
an insulating material as the non-magnetic intermediate layer
disposed between the memory layer 17 and the magnetization-fixed
layer 15 in consideration of the saturated current value of the
selection transistor.
The magnetic tunnel junction (MTJ) element is configured by using
the tunnel insulating layer, such that it is possible to make a
magnetoresistance change ratio (MR ratio) large compared to a case
where a giant magnetoresistive effect (GMR) element is configured
by using a non-magnetic conductive layer, and therefore it is
possible to increase the read-out signal strength.
Particularly, when magnesium oxide (MgO) is used as the material of
the tunnel insulating layer 16, it is possible to make the
magnetoresistance change ratio (MR ratio) large compared to a case
where aluminum oxide, which can be generally used, is used.
In addition, generally, spin injection efficiency depends on the MR
ratio, and the larger the MR ratio, the more spin injection
efficiency is improved, and therefore it is possible to diminish
the magnetization inversion current density.
Therefore, when magnesium oxide is used as the material of the
tunnel insulating layer 16 and the memory layer 17 is used, it is
possible to diminish the threshold write current by spin injection
and therefore it is possible to perform the writing (recording) of
information with a small current. In addition, it is possible to
increase the read-out signal strength.
In this manner, it is possible to diminish the threshold write
current by spin injection by securing the MR ratio (TMR ratio), and
it is possible to perform the writing (recording) of information
with a small current. In addition, it is possible to increase the
read-out signal strength.
As described above, in a case where the tunnel insulating layer 16
is formed of the magnesium oxide (MgO) film, it is desirable that
the MgO film be crystallized and therefore a crystal orientation be
maintained in (001) direction.
In addition, in this embodiment, in addition to a configuration
formed of the magnesium oxide, the insulating layer 16 as an
intermediate layer disposed between the memory layer 17 and the
magnetization-fixed layer 15 may be configured by using, for
example, various insulating materials, dielectric materials, and
semiconductors such as aluminum oxide, aluminum nitride, SiO.sub.2,
Bi.sub.2O.sub.3, MgF.sub.2, CaF, SrTiO.sub.2, AlLaO.sub.3, and
Al--N--O.
An area resistance value of the tunnel insulating layer 16 is
necessary to be controlled to several tens .OMEGA..mu.m2 or less in
consideration of the viewpoint of obtaining a current density
necessary for inverting the magnetization direction of the memory
layer 17 by spin injection.
In the tunnel insulating layer 16 formed of the MgO film, to retain
the area resistance value within the above-described range, it is
necessary to set the film thickness of the MgO film to 1.5 nm or
less.
In addition, it is desirable to make the memory element small to
easily invert the magnetization direction of the memory layer 17
with a small current.
Therefore, preferably, the area of the memory element is set to
0.01 .mu.m2 or less.
In addition, in this embodiment, the memory layer 17 may be formed
by directly laminating another ferromagnetic layer having a
different composition. In addition, a ferromagnetic layer and a
soft magnetic layer may be laminated, or a plurality of
ferromagnetic layers may be laminated through a soft magnetic layer
or a non-magnetic layer interposed therebetween. Even in the case
of laminating in this manner, an effect may be obtained.
Particularly, in a case where the memory layer 17 is configured by
laminating the plurality of ferromagnetic layers through the
non-magnetic layer, it is possible to adjust the strength of
interaction between the ferromagnetic layers, such that even when
even when the dimensions of the memory element is under sub-micron,
there is obtained an effect of controlling magnetization inversion
current not to be large. As a material of the non-magnetic layer in
this case, Ru, Os, Re, Ir, Au, Ag, Cu, Al, Bi, Si, B, C, Cr, Ta,
Pd, Pt, Zr, Hf, W, Mo, Nb, or an alloy thereof may be used.
It is desirable that the magnetization-fixed layer 15 and the
memory layer 17 have a unidirectional anisotropy. In addition, it
is preferable that the film thickness of each of the
magnetization-fixed layer 15 and the memory layer 17 be 0.5 to 30
nm.
Other configuration of the memory element may be the same as the
configuration of a memory element that records information by spin
injection in the related art.
The magnetization-fixed layer 15 may be configured in such a manner
that the magnetization direction is fixed by only a ferromagnetic
layer or by using an anti-ferromagnetic combination of an
anti-ferromagnetic layer and a ferromagnetic layer.
In addition, the magnetization-fixed layer 15 may be configured by
a single layer of a ferromagnetic layer, or a ferri-pin structure
in which a plurality of ferromagnetic layers are laminated through
a non-magnetic layer.
As a material of the ferromagnetic layer making up the
magnetization-fixed layer 15 of the laminated ferri-pin structure,
Co, CoFe, CoFeB, or the like may be used. In addition, as a
material of the non-magnetic layer, Ru, Re, Ir, Os, or the like may
be used.
As a material of the anti-ferromagnetic layer, a magnetic material
such as an FeMn alloy, a PtMn alloy, a PtCrMn alloy, an NiMn alloy,
an IrMn alloy, NiO, and Fe2O3 may be exemplified.
In addition, a magnetic characteristic may be adjusted by adding a
non-magnetic element such as Ag, Cu, Au, Al, Si, Bi, Ta, B, C, O,
N, Pd, Pt, Zr, Hf, Ir, W, Mo, and Nb to the above-describe magnetic
materials, or in addition to this, various physical properties such
as a crystalline structure, a crystalline property, a stability of
a substance, or the like may be adjusted.
In addition, in relation to a film configuration of the memory
element, the memory layer 17 may be disposed at the lower side of
the magnetization-fixed layer 15, or at the upper side thereof, and
in any disposition, there is no problem at all. In addition, there
is no problem at all in a case where the magnetization-fixed layer
15 is disposed at the upper side and the lower side of the memory
layer 17, so-called dual-structure.
In addition, as a method of reading-out information recorded in the
memory layer 17 of the memory element, a magnetic layer that
becomes a reference for the information is provided on the memory
layer 17 of the memory element through a thin insulating film, and
the reading-out may be performed by a ferromagnetic tunnel current
flowing through the insulating layer 16, or the reading-out may be
performed by a magnetoresistive effect.
In addition, in this embodiment, at least an interface, which comes
into contact with the memory layer 17, of the cap layer 18 is
formed of a Ta film. For example, the cap layer 18 may be formed by
only a Ta film having a predetermined film thickness, or a
non-magnetic layer such as an Ru film and a Pt film may be
laminated on the Ta film.
Since the Ta film of the cap layer 18 comes into contact with the
memory layer 17, perpendicular magnetic anisotropy is applied to
the memory layer 17. Due to this, it is easy for the memory layer
17 to become a perpendicular magnetization film. Therefore, it is
more advantageous with regard to decrease in the write current and
with regard to thermal stability.
2. Configuration of Embodiment
Subsequently, a specific configuration of this embodiment will be
described.
As an embodiment, a schematic configuration diagram (perspective
view) of a memory device is shown in FIG. 1.
This memory device includes a memory element 3, which can retain
information at the magnetization state, disposed in the vicinity of
an intersection of two kinds of address interconnects (for example,
a word line and a bit line) that are perpendicular to each
other.
Specifically, a drain region 8, a source region 7, and a gate
electrode 1 that make up a selection transistor that selects each
memory cell are formed in a portion separated by an element
separation layer 2 of a semiconductor substrate 10 such as a
silicon substrate, respectively. Among them, the gate electrode 1
also functions as one side address interconnect (for example, a
word line) that extends in the front-back direction in the
drawing.
The drain region 8 is formed commonly with left and right selection
transistors in the drawing, and an interconnect 9 is connected to
the drain region 8.
The memory element 3 is disposed between the source region 7, and
the other side address interconnect (for example, a bit line) 6
that is disposed at the upper side and extends in the left-right
direction in the drawing. This memory element 3 has a memory layer
including a ferromagnetic layer whose magnetization direction is
inverted by spin injection.
In addition, the memory element 3 is disposed in the vicinity of an
intersection of two kinds of address interconnects 1 and 6.
The memory element 3 is connected to the bit line 6 and the source
region 7 through upper and lower contact layers 4,
respectively.
In this manner, a current flows into the memory element 3 in the
perpendicular direction thereof through the two kind of address
interconnects 1 and 6, and the magnetization direction of the
memory layer may be inverted by a spin injection.
In addition, a cross-sectional view of the memory element 3 of the
memory device according to this embodiment is shown in FIG. 2.
In the memory element 3, an underlying layer 14, the
magnetization-fixed layer 15, the insulating layer 16, the memory
layer 17, and the cap layer 18 are laminated in this order from a
lower layer side.
In this case, the magnetization-fixed layer 15 is provided at a
lower layer in relation to a memory layer 17 in which the
magnetization direction of a magnetization M17 is inverted by a
spin injection.
In regard to the spin injection type memory, "0" and "1" of
information are defined by a relative angle between the
magnetization M17 of the memory layer 17 and a magnetization M15 of
the magnetization-fixed layer 15.
An insulating layer 16 that serves as a tunnel barrier layer
(tunnel insulating layer) is provided between the memory layer 17
and the magnetization-fixed layer 15, and therefore an MTJ element
is configured by the memory layer 17 and the magnetization-fixed
layer 15.
In addition, an underlying layer 14 is formed under the
magnetization-fixed layer 15, and a cap layer 18 is formed on the
memory layer 17.
The memory layer 17 is formed of a ferromagnetic material having a
magnetic moment in which the direction of a magnetization M17 is
freely changed in a direction perpendicular to a film face. The
magnetization-fixed layer 15 is formed of a ferromagnetic material
having a magnetic moment in which a magnetization M15 is fixed in
the direction perpendicular to the film face.
The storage of information is performed by a magnetization
direction of the memory layer 17 having a unidirectional
anisotropy. The writing of information is performed by applying a
current in the direction perpendicular to the film face and by
causing a spin torque magnetization inversion. In this way, the
magnetization-fixed layer 15 is provided at a lower layer in
relation to the memory layer 17 in which the magnetization
direction is inverted by the spin injection, and serves as a
reference for the memory information (magnetization direction) of
the memory layer 17.
In this embodiment, Co--Fe--B is used for the memory layer 17 and
the magnetization-fixed layer 15.
The magnetization-fixed layer 15 serves as the reference for the
information, such that it is necessary that the magnetization
direction does not vary, but it is not necessarily necessary to be
fixed in a specific direction. The magnetization-fixed layer 15 may
be configured in such a manner that migration becomes more
difficult than in the memory layer 17 by making a coercive force
large, by making the film thickness large, or by making a damping
constant large compared to the memory layer 17.
In the case of fixing the magnetization, an anti-ferromagnetic
material such as PtMn and IrMn may be brought into contact with the
magnetization-fixed layer 15, or a magnetic material brought into
contact with such an anti-ferromagnetic material may be
magnetically combined through a non-magnetic material such as Ru,
and thereby the magnetization-fixed layer 15 may be indirectly
fixed.
In this embodiment, particularly, a composition of the memory layer
17 of the memory element 3 is adjusted such that a magnitude of an
effective diamagnetic field which the memory layer 17 receives
becomes smaller than the saturated magnetization amount Ms of the
memory layer 17.
That is, a composition of a ferromagnetic material Co--Fe--B of the
memory layer 17 is selected, and the magnitude of the effective
diamagnetic field which the memory layer 17 receives is made to be
small, such that the magnitude of the effective diamagnetic field
becomes smaller than the saturated magnetization amount Ms of the
memory layer 17.
In addition, in this embodiment, in a case where the insulating
layer 16 that is an intermediate layer is formed of a magnesium
oxide (MgO) layer. In this case, it is possible to make a
magnetoresistive change ratio (MR ratio) high.
When the MR ratio is made to be high as described above, the spin
injection efficiency is improved, and therefore it is possible to
diminish a current density necessary for inverting the direction of
the magnetization M17 of the memory layer 17.
In this embodiment, at least an interface, which comes into contact
with the memory layer 17, of the cap layer 18 is formed of a Ta
film.
As a material of the cap layer 18 that is laminated on the memory
layer 17, Ta is used, and the Ta film comes into contact with the
memory layer 17, such that perpendicular magnetization is applied
to the memory layer 17. Therefore, it is easy to realize a
perpendicular magnetization-type MTJ.
The memory element 3 of this embodiment can be manufactured by
continuously forming from the underlying layer 14 to the cap layer
18 in a vacuum apparatus, and then by forming a pattern of the
memory element 3 by a processing such as a subsequent etching or
the like.
According to the above-described embodiment, the memory layer 17 of
the memory element 3 is configured in such a manner that the
magnitude of the effective diamagnetic field that the memory layer
17 receives is smaller than the saturated magnetization amount Ms
of the memory layer 17, such that the diamagnetic field that the
memory layer 17 receives is decreased, and it is possible to
diminish an amount of the write current necessary for inverting the
direction of the magnetization M17 of the memory layer 17.
On the other hand, since the amount of the write current may be
diminished even when the saturated magnetization amount Ms of the
memory layer 17 is not diminished, it is possible to sufficiently
secure the saturated magnetization amount Ms of the memory layer 17
and therefore it is possible to sufficiently secure the thermal
stability of the memory layer 17.
As described above, since it is possible to sufficiently secure the
thermal stability that is an information retaining ability, it is
possible to configure the memory element 3 excellent in a
characteristic balance.
In this manner, an operation error is removed and an operation
margin of the memory element 3 is sufficiently obtained, such that
it is possible to stably operate the memory element 3.
Accordingly, it is possible to realize a memory device that
operates stably with high reliability.
In addition, the Ta film of the cap layer 18 comes into contact
with the memory layer 17, and therefore perpendicular magnetic
anisotropy is applied to the memory layer 17, such that it is easy
for the memory layer 17 to become a perpendicular magnetization
film and it is advantageous with regard to decrease in a write
current and with regard to thermal stability.
In addition, the write current is diminished, such that it is
possible to diminish the power consumption when performing the
writing into the memory element 3. Therefore, it is possible to
diminish the power consumption of the entirety of the memory device
in which a memory cell is configured by the memory element 3 of
this embodiment.
Therefore, in regard to the memory device including the memory
element 3 capable of realizing a memory device that is excellent in
information retaining ability, has high reliability, and operates
stably, it is possible to diminish the power consumption in a
memory device including the memory element.
In addition, the memory device that includes a memory element 3
shown in FIG. 2 and has a configuration shown in FIG. 1 has an
advantage in that a general semiconductor MOS forming process may
be applied when the memory device is manufactured.
Therefore, it is possible to apply the memory device of this
embodiment as a general purpose memory.
3. Experiment
Here, in regard to the configuration of the memory element of this
embodiment, by specifically selecting the material of the
ferromagnetic layer making up the memory layer 17, the magnitude of
the effective diamagnetic field that the memory layer 17 receives
was adjusted, and thereby a sample of the memory element 3 was
manufactured, and then characteristics thereof was examined.
In an actual memory device, as shown in FIG. 1, a semiconductor
circuit for switching or the like present in addition to the memory
element 3, but here, the examination was made on a wafer in which
only the memory element is formed for the purpose of investigating
a magnetization inversion characteristic of the memory layer
17.
In addition, in the following experiments 1 to 4, investigation is
made into a configuration where a magnitude of the effective
diamagnetic field which the memory layer 17 receives is made to be
small, and thereby the magnitude of an effective diamagnetic field
which the memory layer receives is smaller than a saturated
magnetization amount Ms of the memory layer 17, by selecting a
composition of the ferromagnetic material, that is, Co--Fe--B of
the memory layer 17.
In addition, in experiment 5, investigation is made into a material
of the cap layer 18.
Experiment 1
A thermal oxide film having a thickness of 300 nm was formed on a
silicon substrate having a thickness of 0.725 mm, and the memory
element 3 having a configuration shown in FIG. 2 was formed on the
thermal oxide film.
Specifically, in regard to the memory element 3 shown in FIG. 2, a
material and a film thickness of each layer were selected as
described below. Underlying layer 14: Laminated film of a Ta film
having a film thickness of 10 nm and a Ru film having a film
thickness of 25 nm Magnetization-fixed layer 15: CoFeB film having
a film thickness of 2.5 nm Tunnel insulating layer 16: Magnesium
oxide film having a film thickness of 0.9 nm Memory layer 17: CoFeB
film having the same composition as that of the magnetization-fixed
layer Cap layer 18: Laminated film of a Ta film having a film
thickness of 3 nm, a Ru film having a thickness of 3 nm, and a Ta
film having a thickness of 3 nm
Each layer was selected as described above, a Cu film (not shown)
having a film thickness of 100 nm (serving as a word line described
below) was provided between the underlying layer 14 and the silicon
substrate.
In the above-described configuration, the ferromagnetic layer of
the memory layer 17 was formed of a ternary alloy of Co--Fe--B, and
a film thickness of the ferromagnetic layer was fixed to 2.0
nm.
Each layer other than the insulating layer 16 formed of a magnesium
oxide film was formed using a DC magnetron sputtering method.
The insulating layer 16 formed of the magnesium oxide (MgO) film
was formed using a RF magnetron sputtering method.
In addition, after forming each layer of the memory element 3, a
heating treatment was performed in a magnetic field heat treatment
furnace.
Next, after masking a word line portion by a photolithography, a
selective etching by Ar plasma was performed with respect to a
laminated film other than the word line portion, and thereby the
word line (lower electrode) was formed.
At this time, a portion other than the word line was etched to the
depth of 5 nm in the substrate.
Then, a mask of a pattern of the memory element 3 by an electron
beam drawing apparatus was formed, a selective etching was
performed with respect to the laminated film, and thereby the
memory element 3 was formed. A portion other than the memory
element 3 was etched to a portion of the word line immediately over
the Cu layer.
In addition, in the memory element for the characteristic
evaluation, it is necessary to make a sufficient current flow to
the memory element so as to generate a spin torque necessary for
the magnetization inversion, such that it is necessary to suppress
the resistance value of the tunnel insulating layer. Therefore, a
pattern of the memory element 3 was set to an elliptical shape
having a short axis of 0.09 .mu.m.times.a long axis of 0.18 .mu.m,
and an area resistance value (.OMEGA..mu.m.sup.2) of the memory
element 3 was set to 20 .OMEGA..mu.m.sup.2.
Next, a portion other than the memory element 3 was insulated by
sputtering Al2O3 to have a thickness of substantially 100 nm.
Then, a bit line serving as an upper electrode and a measurement
pad were formed by using photolithography.
In this manner, a sample of the memory element 3 was
manufactured.
By the above-described manufacturing method, each sample of the
memory element 3 in which a composition of Co--Fe--B alloy of the
ferromagnetic layer of the memory layer 17 was changed was
manufactured.
In the composition of the Co--Fe--B alloy, a composition ratio of
CoFe and B was fixed to 80:20, and a composition ratio of Co in
CoFe, that is, x(atomic %) was changed to 90%, 80%, 70%, 60%, 50%,
40%, 30%, 20%, 10%, and 0%.
With respect to each sample of the memory element 3 manufactured as
described above, a characteristic evaluation was performed as
described below.
Before the measurement, it was configured to apply a magnetic field
to the memory element 3 from the outside to control an inversion
current in such a manner that a value in a plus direction and a
value in a minus direction to be symmetric to each other.
In addition, a voltage applied to the memory element 3 was set up
to 1 V within a range without breaking down the insulating layer
16.
Measurement of Saturated Magnetization Amount
The saturated magnetization amount Ms was measured by a VSM
measurement using a Vibrating Sample Magnetometer.
Measurement of Effective Diamagnetic Field
As a sample for measuring an effective diamagnetic field, in
addition to the above-described sample of the memory element 3, a
sample in which each layer making up the memory element 3 was
formed was manufactured and then the sample was processed to have a
planar pattern of 20 mm.times.20 mm square.
In addition, a magnitude M.sub.effective of an effective
diamagnetic field was obtained by FMR (Ferromagnetic Resonance)
measurement.
A resonance frequency fFMR, which is obtained by the FMR
measurement, with respect to arbitrary external magnetic field
H.sub.ex is given by the following equation (3). f.sub.FMR=.gamma.'
{square root over (4.pi.M.sub.effective(H.sub.K+H.sub.ex))} (3)
Here, M.sub.effective in the equation (3) may be expressed by 4.pi.
M.sub.effective=4.pi. Ms-H.perp. (H.perp.: anisotropy field in a
direction perpendicular to a film face).
Measurement of Inversion Current Value and Thermal Stability
An inversion current value was measured for the purpose of
evaluating the writing characteristic of the memory element 3
according to this embodiment.
A current having a pulse width of 10 .mu.s to 100 ms is made to
flow to the memory element 3, and then a resistance value of the
memory element 3 was measured.
In addition, the amount of current that flows to the memory element
3 was changed, and then a current value at which a direction of the
magnetization M17 of the memory layer 17 of the memory element 3
was inverted was obtained. A value obtained by extrapolating a
pulse width dependency of this current value to a pulse width 1 ns
was set to the inversion current value.
In addition, the inclination of a pulse width dependency of the
inversion current value corresponds to the above-described index A
of the thermal stability of the memory element 3. The less the
inversion current value is changed (the inclination is small) by
the pulse width, the more the memory element 3 is strengthened
against thermal disturbance.
In addition, twenty memory elements 3 with the same configuration
were manufactured to take variation in the memory element 3 itself
into consideration, the above-described measurement was performed,
and an average value of the inversion current value and the index
.DELTA. of the thermal stability were obtained.
In addition, an inversion current density Jc0 was calculated from
the average value of the inversion current value obtained by the
measurement and an area of the planar pattern of the memory element
3.
In regard to each sample of the memory element 3, a composition of
Co--Fe--B alloy of the memory layer 17, measurement results of the
saturated magnetization amount Ms and the magnitude M.sub.effective
of the effective diamagnetic field, and a ratio M.sub.effective/Ms
of effective diamagnetic field to the saturated magnetization
amount were shown in Table 1. Here, an amount x of Co of Co--Fe--B
alloy of the memory layer 17 described in Table 1 was expressed by
an atomic %.
TABLE-US-00001 TABLE 1 Ms(emu/cc) Meffctive(emu/cc) Meffective/Ms
(Co.sub.90Fe.sub.10).sub.80--B.sub.20 960 1210 1.26
(Co.sub.80Fe.sub.20).sub.80--B.sub.20 960 1010 1.05
(Co.sub.70Fe.sub.30).sub.80--B.sub.20 1040 900 0.87
(Co.sub.60Fe.sub.40).sub.80--B.sub.20 1200 830 0.69
(Co.sub.50Fe.sub.50).sub.80--B.sub.20 1300 690 0.53
(Co.sub.40Fe.sub.60).sub.80--B.sub.20 1300 500 0.38
(Co.sub.30Fe.sub.70).sub.80--B.sub.20 1260 390 0.31
(Co.sub.20Fe.sub.80).sub.80--B.sub.20 1230 360 0.29
(Co.sub.10Fe.sub.90).sub.80--B.sub.20 1200 345 0.29
Fe.sub.80--B.sub.20 1160 325 0.28
From the table 1, in a case where the amount x of Co in
(Co.sub.xFe.sub.100-x).sub.80B.sub.20 was 70% or less, the
magnitude of the effective diamagnetic field (M.sub.effective) was
smaller than the saturated magnetization amount Ms, that is, the
ratio of M.sub.effective/Ms in a case where the amount x of Co was
70% or less became a value less than 1.0.
In addition, it was confirmed that the more the amount x of Co
decreased, the larger the difference between M.sub.effective and
Ms.
A measurement result of the inversion current value was shown in
FIG. 3, and a measurement result of the index of the thermal
stability was shown in FIG. 4.
FIG. 3 shows a relationship between the amount x (content in CoFe;
atomic %) of Co in the Co--Fe--B alloy of the memory layer 17 and
the inversion current density Jc0 obtained from the inversion
current value.
FIG. 4 shows a relationship between an amount x (content in CoFe;
atomic %) of Co in the Co--Fe--B alloy of the memory layer 17 and
the index .DELTA.(KV/k.sub.BT) of the thermal stability.
As can be seen from FIG. 3, as the amount x of Co decreases, the
inversion current density Jc0 decreases.
This is because in a case where the amount x of Co becomes small,
the saturated magnetization amount Ms increases, but the effective
diamagnetic field M.sub.effective decreases, and therefore the
product of them Ms.times.M.sub.effective becomes small.
As can be seen from FIG. 4, as the amount x of Co decreased, the
index .DELTA.(=KV/k.sub.BT) of the thermal stability increased, and
in a case where the amount x of Co became more or less small to a
degree, the index .DELTA. of the thermal stability became stable to
a large value.
This well corresponds to a change that is expected from the
measurement result of the saturated magnetization amount Ms shown
in Table 1 and a tendency where the index .DELTA. of the thermal
stability from the equation (2) is proportional to the saturated
magnetization amount Ms.
As was clear from the results of Table 1, FIGS. 3 and 4, in a
composition where the amount x of Co was 70% or less and the
effective diamagnetic field M.sub.effective was less than the
saturated magnetization amount Ms, it was possible to diminish the
inversion current value Jc0 with a high thermal stability
maintained, without using a method in which Ms was decreased and
therefore the thermal stability was sacrificed.
Experiment 2
As can be seen from the Experiment 1, in the case of
(Co.sub.xFe.sub.100-x).sub.80B.sub.20, it was possible to diminish
the inversion current value Jc0 with a high thermal stability
maintained in a composition where the amount x of Co was 70% or
less.
Therefore, in experiment 2, an effect on a ratio of Co and Fe, and
the M.sub.effective/Ms, which was caused by an amount z of B, was
examined by using a memory layer 17 having a composition
(Co.sub.70Fe.sub.30).sub.80B.sub.z and a composition
(Co.sub.80Fe.sub.20).sub.80B.sub.z. The details of a sample were
substantially the same as those in the experiment 1.
Table 2 shows compositions of CoFeB alloy in which the amount z of
B was set to 5 to 40% in (Co.sub.70Fe.sub.30).sub.100-ZB.sub.z,
results of measurement of the saturated magnetization amount Ms and
the magnitude M.sub.effective the effective diamagnetic field, and
a ratio M.sub.effective/Ms of the saturated magnetization amount
and the magnitude of the effective diamagnetic field.
In addition, Table 3 shows compositions of CoFeB alloy in which the
amount z (atomic %) of B was similarly set to 5 to 40% in
(Co.sub.80Fe.sub.20).sub.100-zB.sub.z, and a ratio
M.sub.effective/Ms of the saturated magnetization amount Ms and the
magnitude M.sub.effective of the effective diamagnetic field.
TABLE-US-00002 TABLE 2 Ms(emu/cc) Meffective(emu/cc) Meffective/Ms
(Co.sub.70Fe.sub.30).sub.95--B.sub.5 1310 1090 0.83
(Co.sub.70Fe.sub.30).sub.90--B.sub.10 1250 1080 0.89
(Co.sub.70Fe.sub.30).sub.80--B.sub.20 1040 900 0.87
(Co.sub.70Fe.sub.30).sub.70--B.sub.30 820 730 0.89
(Co.sub.70Fe.sub.30).sub.60--B.sub.40 450 690 1.53
TABLE-US-00003 TABLE 3 Ms(emu/cc) Meffective(emu/cc) Meffective/Ms
(Co.sub.80Fe.sub.20).sub.95--B.sub.5 1250 1280 1.02
(Co.sub.80Fe.sub.20).sub.90--B.sub.10 1100 1140 1.04
(Co.sub.80Fe.sub.20).sub.80--B.sub.20 960 1010 1.05
(Co.sub.80Fe.sub.20).sub.70--B.sub.30 750 890 1.19
(Co.sub.80Fe.sub.20).sub.60--B.sub.40 430 690 1.60
From the results of Table 2, it can be confirmed that in a case
where the ratio of Co and Fe was set to 70/30 like
(Co.sub.70Fe.sub.30).sub.100-ZB.sub.z, the magnitude
M.sub.effective of the effective diamagnetic field was smaller than
the saturated magnetization amount Ms in compositions other than a
composition where the amount z of B was 40 atomic %.
From the results of Table 3, it can be confirmed that in a case
where the ratio of Co and Fe was set to 80/20 like
(Co.sub.80Fe.sub.20).sub.100-ZB.sub.z, the magnitude
M.sub.effective of the effective diamagnetic field was larger than
the saturated magnetization amount Ms in all compositions.
From the results of the above-described Tables 1 to 3, it was
revealed that in a case where the amount z of B is within a range
of 30 atomic % or less, a magnitude correlation of the saturated
magnetization amount Ms and the magnitude M.sub.effective of the
effective diamagnetic field is determined by the ratio of Co and
Fe.
Therefore, a composition of the Co--Fe--B alloy where the magnitude
M.sub.effective of the effective diamagnetic field is less than the
amount of the saturated magnetization Ms in the memory layer 17 is
as follows: (Co.sub.x--Fe.sub.y).sub.100-z--B.sub.z,
Here, 0.ltoreq.Co.sub.x.ltoreq.70,
30.ltoreq.Fe.sub.y.ltoreq.100,
0<B.sub.z.ltoreq.30.
Experiment 3
In a spin injection type memory of the Gbit class, it was assumed
that the size of the memory element is 100 nm.phi. or less.
Therefore, in experiment 3, the thermal stability was evaluated by
using a memory element having the size of 50 nm.phi..
In the composition of Co--Fe--B alloy, a composition ratio (atomic
%) of CoFe and B was fixed to 80:20, and a composition ratio x
(atomic %) of Co in CoFe was changed to 90%, 80%, 70%, 60%, 50%,
40%, 30%, 20%, 10%, and 0%.
The details of the sample other than the sample size were
substantially the same as those in the experiment 1.
In a case where the size of the memory element 3 was 50 nm.phi., a
relationship between an amount of Co (content in CoFe; atomic %) in
the Co--Fe--B alloy, and the index .DELTA.(KV/k.sub.BT) of thermal
stability were shown in FIG. 5.
As can be seen from FIG. 5, when the element size was 50 nm.phi.,
Co--Fe--B alloy composition dependency of the thermal stability
index .DELTA. was largely varied from the Co--Fe--B alloy
composition dependency of .DELTA. obtained in the elliptical memory
element having a short axis of 0.09 .mu.m.times.a long axis of 0.18
.mu.m shown in FIG. 4.
According to FIG. 5, the high thermal stability was maintained only
in the case of Co--Fe--B alloy composition where Fe is 60 atomic %
or more.
As a result of various reviews, it was clear that the reason why
the Co--Fe--B alloy containing Fe of 60 atomic % or more shows the
high thermal stability .DELTA. in the extremely minute memory
element was revealed to be because the magnetization of the
Co--Fe--B alloy faced a direction perpendicular to a film face.
The reason why the magnetization of the Co--Fe--B alloy faces the
direction perpendicular to the film face is considered to be
because of a composition in which the magnitude M.sub.effective of
the effective diamagnetic field is significantly smaller than the
saturated magnetization amount Ms.
In addition, the reason why the thermal stability is secured even
in the case of the extremely minute element of a perpendicular
magnetization film is related to Hk (effective anisotropy field) in
the equation (2), and Hk of the perpendicular magnetization film
becomes a value significantly larger than that in the in-plane
magnetization film. That is, in the perpendicular magnetization
film, due to an effect of large Hk, it is possible to maintain a
high thermal stability .DELTA. even in the case of the extremely
minute element not capable of securing a sufficient thermal
stability .DELTA. in the in-plane magnetization film.
From the above-described experiment results, in regard to the
Co--Fe--B alloy having a composition of
(Co.sub.xFe.sub.100-x).sub.80B.sub.20, in a case where the amount
of Fe.sub.100-x is 60% or more, this alloy may be said to be
suitable for the memory device of the Gbit class using the spin
injection.
Experiment 4
As can be seen from the above-described experiment 3, in a case the
amount of Fe was 60 or more in the Co--Fe--B alloy having a
composition of (Co.sub.xFe.sub.100-x).sub.80B.sub.20, this alloy
was suitable for the memory device of the Gbit class using the spin
injection. In experiment 4, a memory element having the size of 50
nm.phi. was manufactured using the Co--Fe--B alloy containing B in
an amount of 5 to 30 atomic %, and the thermal stability was
evaluated.
The details other than the element size were substantially the same
as those in the experiment 1.
A relationship between the index .DELTA.(KV/k.sub.BT) of the
thermal stability and the Co--Fe--B alloy having a composition
(Co.sub.xFe.sub.100-x).sub.100-zB.sub.z in which an amount x of Co
was 50, 40, 30, 20, 10, and 0, and an amount z of B was 5, 10, 20,
and 30 was shown in Table 4.
TABLE-US-00004 TABLE 4 (Co.sub.50--Fe.sub.50).sub.100-z--B.sub.z
(Co.sub.40--Fe.sub.60).sub.100-- z--B.sub.z
(Co.sub.30--Fe.sub.70).sub.100-z--B.sub.z (Co.sub.20--Fe.sub.80-
).sub.100-z--B.sub.z (Co.sub.10--Fe.sub.90).sub.100-z--B.sub.z
Fe.sub.100-- z--B.sub.z B.sub.z = 5 19 40 42 42 43 44 atomic %
B.sub.z = 10 20 41.5 43 44 44 45 atomic % B.sub.z = 20 20 43 44 45
46 46 atomic % B.sub.z = 30 21 45 47 48 48 48 atomic %
As can be seen from Table 4, the thermal stability .DELTA. in all
compositions except that a case where the amount x of Co was 50,
and the amount z of B was 5 to 30 was maintained to be large.
That is, as is the case with the result of the experiment 4, it was
revealed that the amount x of Co of 50 and 60 became a boundary
line for securing high thermal stability in an extremely minute
element corresponding to the spin injection type memory of the Gbit
class.
Therefore, from the above-described result, it was revealed that
the Co--Fe--B alloy of the memory layer 17 was suitable for
manufacturing the spin injection type memory of the Gbit class in
the following composition:
(Co.sub.x--Fe.sub.y).sub.100-z--B.sub.z,
Here, 0.ltoreq.Co.sub.x.ltoreq.40,
60.ltoreq.Fe.sub.y.ltoreq.100,
0<B.sub.z.ltoreq.30.
In addition, in regard to the Co--Fe--B alloy, in a composition
where the ratio of Fe was great in Co and Fe, the difference
between the M.sub.effective and Ms becomes large, and this alloy is
apt to be magnetized, and therefore it is easy to secure thermal
stability.
Therefore, in a case where the capacity of the magnetic memory
increases and the size of the memory element 3 decreases, it is
easy to secure thermal stability in the Co--Fe--B alloy containing
a large amount of Fe.
Therefore, for example, in consideration of a situation in which
the spin injection type magnetic memory of the Gbit class is
realized by the memory layer 17 in which the amount y of Fe is 60,
and the size thereof is 70 nm.phi., it is preferable that whenever
the diameter of the memory element 3 decreases by 5 nm.phi., the
amount y of Fe in the Co--Fe--B alloy increase by a value of 5.
For example, in the case of the
(Co.sub.x--Fe.sub.y).sub.100-z--B.sub.z, the amount y of Fe is set
in such a manner that an atomic % as a content in CoFe is 65%, 70%,
75%, 80%, . . . (in terms of the amount x of Co, 35%, 30%, 25%,
20%, . . . ), and this is a more appropriate example to correspond
to the size reduction of the memory element.
Experiment 5
Next, various samples in which a material of the cap layer 18 was
changed was manufactured, and an investigation was made into an
appropriate structure of the cap layer 18.
A thermal oxide film having the thickness of 300 nm was formed on a
silicon substrate having the thickness of 0.725 mm, and a magnetic
multi-layered film shown in FIG. 6A was formed on the thermal oxide
film.
In this case, this sample was configured by a Ta film (5 nm), an Ru
film (10 nm), a Ta film (5 nm), a
(Co.sub.20Fe.sub.80).sub.80B.sub.20 film (1 nm), an MgO film (1
nm), a (Co.sub.20Fe.sub.80).sub.80B.sub.20 film (1.3 nm), and a
layer formed of a material X in this order from an underlying film
side.
In this case, this configuration is a magnetic multi-layered film
model in which from a lower side, the Ta film, the Ru film, and the
Ta film correspond to the underlying layer 14, the lower side
(Co.sub.20Fe.sub.80).sub.80B.sub.20 film corresponds to the
magnetization-fixed layer 15, the MgO film corresponds to the
insulating layer 16, and the upper side
(Co.sub.20Fe.sub.80).sub.80B.sub.20 film corresponds to the memory
layer 17.
After forming the magnetic multi-layered film, a heating treatment
was performed in a magnetic field heat treatment furnace.
Then, in regard to the layer indicated by the material X, which
corresponds to the cap layer 18, in the drawing, a sample in which
this layer was formed of an Ru film of 5 nm, and a sample in which
this layer was formed of a Ta film of 5 nm were manufactured, and
then a Kerr measurement was performed with respect to each sample
and a magnetic characteristic was examined.
Measurement results with respect a case where the cap layer 18 was
formed of the Ru film (5 nm), and a case where the cap layer 18 was
formed of the Ta film (5 nm) are shown in FIG. 6B.
In the case of the Ru film, a magnetization inversion step has only
one stage (a portion indicated by an arrow A). This is caused by
the lower side CoFeB layer (magnetization-fixed layer 15), and this
shows that the upper side CoFeB layer (memory layer 17) is not
perpendicularly magnetized.
On the other hand, in a case where the cap layer 18 is formed of
the Ta film, the magnetization inversion step has two stages, that
is, a portion indicated by an arrow A and a portion indicated by an
arrow B. The portion indicated by the arrow B shows magnetization
inversion in the memory layer 17.
That is, when the cap layer 18 is formed of the Ta film, the upper
side and lower side CoFeB layers are perpendicularly
magnetized.
As can be seen from this result, when Ta is used for the cap layer,
it is possible to realize a perpendicular magnetization-type
MTJ.
Next, as shown in FIG. 7A, a measurement was performed using
samples in which the cap layer 18 had a laminated structure of a Ta
film and an Ru film was manufactured
In addition, in regard to the cap layer 18, samples in which the Ta
film that brought into contact with the memory layer 17 had a film
thickness of 5 nm, 3 nm, and 1 nm were used.
Measurement results are shown in FIG. 7B.
As the Ta film thickness decreases, a separation (a portion
indicated by an arrow A, and a portion indicated by an arrow B) of
magnetization inversion step becomes clear, and the perpendicular
magnetization of the upper side CoFeB (memory layer 17) is
strengthened.
From these results, it can be seen that it is preferable that the
film thickness of the Ta film of the cap layer 18 be 5 nm or
less.
The cap layer 18 may be formed by further laminating another
material on the Ta film/Ru film.
A measurement was performed with respect to samples in which the
cap layer 18 had a laminated structure of, for example, a Ta film
of 5 nm, an Ru film of 5 nm, and a Ta film of 3 nm as shown in FIG.
8A, and the measurement results as shown in FIG. 8B was
obtained.
In this case, two-stages magnetization inversion is also shown as
portions indicated by arrows A and B, the upper side and lower side
CoFeB layers are perpendicularly magnetized.
In addition, another material to be laminated with the Ta film was
investigated. For example, a case using Pt is shown in FIGS. 9A to
9D.
In a sample shown in FIG. 9A, the cap layer 18 includes a Pt film
of 5 nm, an Ru film of 3 nm, and Ta film of 5 nm. Pt film comes
into contact with the memory layer 17.
In this case, measurement results shown in FIG. 9B were obtained.
From the measurement results, it can be seen that magnetization
inversion shows only one stage (portion indicated by an arrow A),
and the upper side CoFeB layer (memory layer 17) is not
perpendicularly magnetized.
On the other hand, in a sample shown in FIG. 9C, the cap layer 18
includes a Ta film of 1 nm, a Pt film of 5 nm, an Ru film of 5 nm,
and a Ta film of 3 nm. The Ta film comes into contact with the
memory layer 17.
In this case, a measurement result shown in FIG. 9D was obtained.
Here, two-stages (a portion indicated by an arrow A and a portion
indicated by an arrow B) magnetization inversion was shown, and it
was confirmed that the upper side and lower side CoFeB layers (the
magnetization-fixed layer 15 and the memory layer 17) were
perpendicularly magnetized.
From the above-described results of the experiment 5, it can be
understood that the using of Ta film for the cap layer 18 is
suitable for realizing the perpendicular magnetization-type
MTJ.
In addition, it is preferable that the thickness of the Ta film be
5 nm or less, and more preferably, 3 nm or 1 nm.
In addition, even as the cap layer 18 is formed by laminating a
non-magnetic film such as Ru and Pt, there is not problem. However,
it is necessary that at least a face that comes into contact with
the memory layer is formed of a Ta film.
Hereinbefore, the embodiment is described, but the present
application is not limited to the above-described embodiment, and
it is possible to adopt various layer configurations.
For example, in the embodiment, the Co--Fe--B composition of the
memory layer 17 and the magnetization-fixed layer 15 was made to be
the same as each other, but it is not limited to the
above-described embodiment, and various configurations may be made
without departing from the scope.
In addition, the underlying layer 14 may be formed of a single
material, or may be formed by a laminated structure of a plurality
of materials.
In addition, in this embodiment, the magnetization-fixed layer 15
may be formed by a single layer, or may use a laminated ferri-pin
structure including two ferromagnetic layers and a non-magnetic
layer. In addition, a structure in which anti-ferromagnetic film is
applied to the laminated ferri-pin structure film is possible.
In addition, a film configuration of the memory element may be a
configuration in which the memory layer 17 is disposed at an upper
side of the magnetization-fixed layer 15 or a configuration in
which the memory layer 17 is disposed at a lower side.
In addition, this film configuration may be a so-called dual
structure in which the magnetization-fixed layer 15 is disposed at
the upper side and the lower side of the memory layer 17.
It should be understood that various changes and modifications to
the presently preferred embodiments described herein will be
apparent to those skilled in the art. Such changes and
modifications can be made without departing from the spirit and
scope and without diminishing its intended advantages. It is
therefore intended that such changes and modifications be covered
by the appended claims.
* * * * *